That isn't true. Whilst you did find a method, I tried the exact same scenarios posted in your guide and I didn't get many of the results consistantly (90-100% of the time) that you did for the large majority of my findings, that's why my second testing dummy was Sheik because that's what you used. Your first example is that Mewtwo and Sheik are both facing > and she should be sent <, but my tests found that this only happened 70% of the time and that the direction Sheik faced didn't have an effect on it, whilst the direction Mewtwo faced did (referring to nair cancelling here, not full nair).
You also stated that you found the nair to work at the height of Mewtwo's jump to get the opponent to pop in front of Mewtwo, which isn't what I said at all, and I didn't find this to work as consistently as delaying the FF until Mewtwo has spent a few frames starting to fall.
Your thoughts on FH nair being influenced by DI are indeed correct, I got the exact same results you did.
Your nair guide was also quite hard to find as well (though that's no fault of your own, Smashboards looks very cluttered imho), and I think you should spend more time on it, as it's lacking important information which I needed to lab myself. Which is exactly why I posted my findings here.